Defects in mitochondrial energy production contribute to a variety of diseases including congenital heart defects (CHD) in the neonate. CHD occur in approximately 1 % of live human births in the USA and are almost certainly responsible for many more spontaneous abortions. The contribution of mitochondrial mitopathies to fetal heart failure has not been previously investigated. Calcineurin (CNA) signaling through activation of NEAT is associated with changes in myocyte function in cardiac muscle. NEAT activation is sufficient to induce cardiac hypertrophy and re-expression of fetal genes in adult mice. However, a role for NEAT signaling in embryonic heart development and function is not known. In addition, altered gene expression and cardiomyopathies are associated with energy depletion, which contributes to the pathology of adult cardiomyopathy. This proposal will determine whether NFAT signaling is required for energy metabolism during embryonic cardiac development and establish the requirement of NFAT signal activation specifically in cardiac development via rescue of embryonic lethality presented by nfatc3-/- nfatc4-/- embryos. The hypothesis is that NFAT signaling is required for energy metabolism and cardiovascular function during embryonic development. Mitochondrial maturation and function will be analyzed by electron microscopy and enzymatic assays for SDH and CoxC activity in wildtype, nfatc3-/- nfatc4-/-, and alpha- MHC/NFATc4delta317 embryos. RT-PCR from embryonic heart RNA will determine metabolic gene expression. Morphological analysis of embryonic hearts will be performed to link cardiac development with NEAT signal activation and metabolic activity. Genetic rescue of nfatc3-/- nfatc4-/- lethality will be attempted using the alpha- MHC/NFATc4delta317 transgenic mouse line that expresses constitutively active NFATc4 in the embryonic heart. Linkage of NFAT signaling with control of metabolism would be valuable in . the rational design of effective pharmaceutical intervention. In addition, the data derived in this study will facilitate future studies in this critical area of embryonic heart function, metabolism and disease.